Yuanfeng Cui

Previous studies have noted that cities enhance cloud cover, but the mechanisms of urban morphological types on cloud formation remain elusive. Observations of cloud climatology from 44 major U.S. cities show that cloud enhancement increases with the street-canyon aspect ratio and decreases with building density. Here, to explain these observations, we conducted numerical experiments using urban morphology-resolving large-eddy simulations. In these simulations, urban and rural surfaces retain their respective heat-flux differences, while the moisture sources and background atmospheric water vapor are prescribed to be identical, allowing us to isolate the morphological controls on moist convection. Results show that urban morphology influences cloud formation through two mechanisms: taller buildings intensify urban-breeze circulations at the urban-rural interface, while denser buildings, acting as momentum sinks, reduce vertical turbulent transport at the urban core. These vertical motions modify the transport of moisture in the urban atmospheric boundary layer, causing different cloud amounts across different urban morphology. This study highlights the mechanistic link between urban form, vertical motions, and cloud enhancement, thus providing a basis for city-specific boundary-layer convective parameterizations in large-scale weather and climate models.

Anthropogenic heat (AH) emissions in urban environments alter the surface energy budget and significantly influence urban climates. However, these emissions vary greatly in both time and space, leading to considerable uncertainty in their estimation. As remote sensing in the urban environment advances, where the remotely sensed urban surface temperatures are becoming increasingly available, such as those retrieved from satellite observations and thermal cameras. Yet, assimilating these observations into surface energy modeling for AH estimation has not been fully explored. In this study, a model for AH estimation based on the Kalman filter and surface energy balance is developed (KF-SEB model). Urban meteorological data, including air temperature and building surface temperature, are assimilated into the Kalman filter, yielding sensible heat flux and building heat storage. AH is subsequently calculated using the SEB equation. The KF-SEB model is evaluated using a forward model with predefined AH emissions. The forward model employs a simple SEB approach at the building exterior surface and adopts a 1-D heat conduction equation for the wall. The results show that the KF-SEB model accurately captures the magnitude and temporal variation of AH, with reduced uncertainties compared to previous studies. This study offers a novel approach of AH estimation based on urban meteorological data and provides important insights into the feedback between urban microclimates and anthropogenic energy use.

Previous studies have noted that cities enhance cloud cover, but the mechanisms of urban morphological types on cloud formation remain elusive.Considering basic types of urban land cover land use, we then set up idealized large-eddy simulations (LESs) for varying urban morphological types with different building heights and building plan area fractions under free convective conditions. LES results suggest that cloud patterns closely correspond to the vertical transport of moisture. Clouds over urban-rural interface are controlled by the urban breeze circulation (UBC); while clouds over urban core are controlled by vertical velocity variance. Thus, urban morphologies affect cloud formation via two plausible mechanisms: the roughness-induced convergence at the urban-rural interface increases intensity of UBC while roughness as momentum sinks reduces vertical turbulent transport at the urban core. Two conceptual models are proposed to explain the variation of vertical velocity scales with morphologies. Observations across 44 major US cities corroborate LES results: cloud enhancement increases with street-canyon aspect ratio and decreases with building density. This study demonstrates the importance of capturing vertical motions modified by urban roughness for boundary-layer parameterizations. It also underscores the value of synthesizing long-term observations across distinct cities to understand the effect of urban form on boundary-layer processes.

Previous studies have noted that cities enhance cloud cover, but the mechanisms of urban morphological types on cloud formation remain elusive. Observations of cloud climatology from 44 major U.S. cities show that cloud enhancement increases with the street-canyon aspect ratio and decreases with building density. To explain these observations, we conducted numerical experiments using urban morphology-resolving large-eddy simulations. Results show that urban morphology influences cloud formation through two mechanisms: taller buildings intensify urban-breeze circulations at the urban-rural interface, while denser buildings, acting as momentum sinks, reduce vertical turbulent transport at the urban core. These vertical motions modify the transport of moisture in the urban atmospheric boundary layer, causing different cloud amounts across different urban morphology. This study for the first time establishes a mechanistic link between urban form and cloud enhancement. The results also provide a basis for city-specific boundary-layer convective parameterizations in large-scale weather and climate models.

The spatiotemporal characteristics of air temperature and humidity mediated by urban bluespace are investigated using a combination of dense network of climatological observations in a medium-sized US city, computational fluid dynamics and analytical modeling approaches. Both numerical simulation and observational results show that the rate of change of hourly averaged air temperature and humidity at 3.5 m over urban areas peaks two hours after sunset, while it decreases with time monotonically over greenspace, indicating different impacts due to presence of urban lakes. The apparent temperature decreases with distance to lakes in urban area due to higher near-shore humidity. This highlights that urban lakes located near city center can deteriorate the nighttime cooling effects due to elevated humidity. Finally, two analytical models are presented to explain the connection between the surface and air temperature as well as the spatial variation of air temperature and humidity adjacent to the urban lakes. These simplified models with parameters being inferred from the network of measurements have reasonably good performance compared to the observations. Compared to other sophisticated numerical simulations, these analytical models offer an alternative means that is easily accessible for evaluating the efficacy of bluespace on urban nocturnal cooling.